Field of Invention
The present disclosure relates to electrochemical deposition and, more particularly, to self-terminating growth of platinum by electrochemical deposition.
Description of Related Art
Platinum has been used as a key constituent in a number of heterogeneous catalysts. However, because platinum is expensive, its use in the development of alternative energy conversion systems—such as low temperature fuel cells—has been somewhat limited. In the meantime, strategies are being explored to minimize platinum loadings, while also enhancing catalyst performance. The strategies range from alloying to nanoscale engineering of core-shell and related architectures that may involve spontaneous processes such as dealloying and segregation to form platinum-rich surface layers.
Deposition of two-dimensional (2-D) platinum layers is of interest in areas such as thin film electronics, magnetic materials, electrocatalysts, and catalytically active barrier coating for corrosion management. Such two-dimensional deposition is non-trivial because the step-edge barrier to interlayer transport results in roughening or three-dimensional mound formation. The chemical and electronic nature of the Pt films may also be a function of its roughness, thickness and the underlying substrate.
In situ scanning tunnel microscopy (STM) has been used to analyze platinum electrodeposition. When platinum is electrodeposited onto gold at moderate overpotentials, STM reveals how the metal nucleation and growth proceeds on gold. More particularly, STM shows that this nucleation and growth proceeds by formation of three-dimensional clusters at defect sites on single crystal surfaces. At small overpotentials, smooth platinum monolayers may be electrodeposited on gold with a long growth time, e.g., two thousand (2,000) seconds. X-ray scattering may be used to confirm this smoothness. Voltammetric studies may show a potential-dependent transition between two-dimensional islands versus three-dimensional multilayer growth. However, only partial platinum monolayer coverage may be obtained in the two-dimensional growth regime.
There is a need for a process for electrodepositing a platinum monolayer that results in better coverage in the two-dimensional growth regime.
To address these difficulties, surface limited place exchange reactions are being explored. Galvanic displacement of an underpotential deposited metal monolayer, e.g., copper, may occur by the desired platinum group metal, with the exchange resulting in a sub-monolayer coverage of the noble metal. The process may be repeated to form multiple layers using a variant, electrochemical atomic layer epitaxy. This process may require an exchange of electrolytes and some care to control the trapping of less noble metal as a minor alloying constituent within the film. There is a need for a deposition process that addresses these shortcomings.
In addition, a drawback of some prior art underpotential deposition (upd) reactions is that many of such reactions may be reversed. These reversals make it difficult to control deposition processes, especially when considering sub-nanometer scale films. To avoid the reversibility issues, irreversible processes like vapor phase deposition of thin films at low temperatures may be used. Robust additive fabrication schemes may facilitate these irreversible processes. However, a shortcoming of this approach is that kinetic factors may constrain the quality of the resulting films.
There remains a need for a process for depositing high coverage ultrathin (monolayer thick) platinum films and alloys thereof, so that kinetic factors do not constrain the quality of the resulting films.